Continuous elongated members at least partially housed within buried conduit are normally laid on the bed of a body of water to form underwater gas or power lines, which must be protected close to landing docks or in shallow water. In shallow water, it is therefore common practice to bury such members in the bed of the body of water.
For protection in shallow water, continuous elongated members are normally buried directly in a trench dug into the bed of the body of water. Known techniques of doing this are known as pre-trenching and post-trenching. Pre-trenching consists in digging a trench; laying the continuous elongated member inside the trench; and backfilling the trench to bury the continuous elongated member. Post-trenching consists in laying the continuous elongated member on the bed of the body of water; digging a trench in the bed of the body of water underneath the continuous elongated member, so the latter sinks into the trench; and backfilling the trench to bury the continuous elongated member. Since the trench tends to eventually fill up on its own (natural backfilling), the backfilling and burying steps may be omitted.
Drawbacks common to both the above techniques of burying continuous elongated members are the enormous amount of energy required to excavate large masses of the bed of the body of water, and slow trench digging speed.
Pre-trenching has the drawback of requiring a widely flared trench, to prevent the trench from backfilling naturally, by the lateral faces of the trench caving in, before the continuous elongated member is laid. The time lapse between digging the trench and laying the continuous elongated member may be relatively considerable.
Post-trenching poses a similar problem, though to a lesser degree, by having less flared trench walls, and by having to prevent the trench from backfilling before the member reaches the required depth. The length of the open trench may extend to hundreds of meters.
Moreover, both the above techniques call for two separates passes along the burial route: one pass to lay the member, and one to dig the trench.
Certain of the above drawbacks encountered in burying continuous elongated members also apply to burying conduits.